1. Field of the Invention
This invention relates to a high pressure air gun for the generation of seismic energy in water, and more particularly to a high pressure air gun employing a dual sleeve-type shuttle allowing optimized air discharge and acoustic pressure pulse within the shortest possible time.
2. Description of the Prior Art
In marine seismic exploration, a source of acoustic energy is released into the water every few seconds to produce appropriate acoustic waves that propagate from the source into the earth's surface. These acoustic waves, upon contacting the marine floor and subfloor geologic formations, are reflected back to recording instruments having transducers which convert these waves into electrical signals which are then recorded. Analysis of these electronic signals provides insight into the structure of the subsurface geological formations.
There have been many devices utilized for generating these seismic or acoustic waves. Most recently, however, a major marine seismic energy source has been the air gun. (The term "air gun" is intended to encompass an apparatus for dispersing any suitable compressible gaseous fluid such as air, steam, nitrogen, carbon dioxide, gaseous products of combustion and so forth.) These air guns are capable of releasing high pressure air on the order of 2,000 psi to 6,000 psi in the water to create the desired acoustic waves.
The acoustic pulse generated by an air gun is proportional to the bubble velocity formed by the air escaping the air gun. As the air exits the gun ports a bubble is formed. This bubble accelerates outwardly generating the acoustic pressure pulse which creates the seismic wave. As long as the air bubble/water interface is accelerating, the acoustic pulse amplitude is increasing. Once the maximum bubble velocity is reached, then the maximum acoustic pulse is achieved.
Conventional air guns typically include an annular housing having a chamber in which compressed air is stored, and exhaust ports which allow the stored air to escape from the housing. The only moving component of the air gun, except for a solenoid triggering device, is a shuttle valve which, when shifted, permits air to escape from the firing chamber through the exhaust ports in the main housing into the surrounding water. The guns also include a constant source of compressed air through an inlet passage in the housing which supplies the compressed air for the storage chamber and which enters a control chamber to force the shuttle into a closed position over the exhaust ports in the housing. A solenoid valve is used to allow air to flow into a firing chamber opposite the control chamber having a shuttle bearing surface of greater surface area than the bearing surface in the control chamber. This creates unequal pressure on the shuttle, forcing the shuttle in an open position to expose the exhaust ports and allow the compressed air to escape into the surrounding water. When the shuttle is in a prefire or closed position, the air gun is charged and ready for firing. When fired, by activating the solenoid, the compressed air escapes into the water.
A particular air gun of this type is described in Chelminski U.S. Pat. No. 4,472,794. This air gun comprises a housing and a primary chamber within the housing and a movable shuttle control adapted to reciprocate along an annular chamber within the housing between a first position covering exhaust ports in the primary chamber and a second position laterally displaced from the exhaust ports to allow air to escape from the primary chamber.
A constant source of compressed air is supplied to the air gun which initially acts to force the shuttle into the first position blocking the exit of air through the exhaust ports. This is accomplished by compressed air acting against an exposed upper end surface area of the shuttle within the annular chamber. The same compressed air source also supplies pressurized air into a primary chamber and builds the pressure to a predetermined level. A solenoid controlling the flow of compressed air is activated which then causes compressed air to bear against a surface on an exposed lower end of the shuttle creating a pressure differential which causes the shuttle to shift, exposing the exhaust ports. As the shuttle begins to shift the shuttle's entire lower end surface area is exposed to the pressurized air in the primary chamber. This produces a high acceleration opening force, rapidly shifting the shuttle to fully expose the exhaust ports.
In the Chelminski air gun, however, the exhaust area is much smaller than the 360.degree. periphery of the housing. In addition, because the compressed air in the storage chamber fills a volume equaling the entire space within the storage chamber, the overall average path length for the pressurized gas in discharging from the firing chamber through the exhaust ports is greater than would be desired. Furthermore, once the shuttle is released, there is no further control of the shuttle. The shuttle can return to its prefire condition only by virtue of the air in the firing chamber dropping to some predetermined level.
The inventor of the present invention solved many of the problems associated with the Chelminski invention and other prior art in his U.S. Pat. No. 4,623,033. The air gun of that invention includes an annular housing having a primary chamber for storing compressed air where the primary chamber opens 360.degree. around the periphery of the housing, surrounding a center post section of the body. An external sleeve-type shuttle surrounds the body and is movable between a first position covering the 360.degree. opening in a prefire mode and a second position exposing the 360.degree. opening in a fired position. The single shuttle is held in the prefire or first position by pressure from a source of compressed air acting against a bearing surface of the shuttle in a control chamber formed within the body. This source of compressed air also supplies the air for the primary chamber. The shuttle is forced into the second or fired position by a solenoid activated by an electrical signal which causes compressed air to flow into a firing chamber. The resultant force in the firing chamber becomes larger than the holding force of the control chamber, thereby causing the shuttle to abruptly slide along the outside surface of the housing suddenly and continuously opening the 360.degree. port of the primary chamber. Compressed air from the primary chamber is thus released suddenly into the surrounding water.
When the solenoid is deactivated, by removing the electrical signal, the shuttle is then able to move back into the prefire or first position under the influence of the pressure from the control chamber, which pressure is supplied by the continuous supply of compressed air feeding both the control chamber and the primary chamber. In this manner the movement of the shuttle is completely controlled by the solenoid.
A limitation on the performance of conventional air guns is believed to be the rate at which the flow area for the escaping air may be increased. In conventional air guns, as described earlier herein, the rate of the flow area increase is limited by its smaller port size and the rate at which the air gun shuttle may move. Accordingly, the present invention provides a new method and apparatus which uses multiple shuttles to optimize the rate of opening of the flow area to thereby optimize the rate of discharge of the air.